The present invention relates to a display unit and a semiconductor light emitting device. More particularly, the present invention relates to a display unit and a semiconductor light emitting device, which are fabricated using wurtzite type compound semiconductor layers such as GaN based semiconductor layers.
Conventionally, a semiconductor light emitting device of this type has been fabricated by forming a low temperature buffer layer overall on a sapphire substrate, forming an n-side contact layer made from Si-doped GaN thereon, and stacking, on the n-side contact layer, an n-side cladding layer made from Si-doped GaN, an active layer made from Si-doped InGaN, a p-side cladding layer made from Mg-doped AlGaN, and a p-side contact layer made from Mg-doped GaN. Commercial products of semiconductor light emitting devices having the above-described structure have been fabricated on a large scale. Examples of these commercial products are light emitting diodes and semiconductor lasers allowing emission of light of blue and green in a wavelength range of 450 nm to 530 nm.
A sapphire substrate has often been used for growing gallium nitride thereon. However, dislocations may occur in the crystal, at a high density, due to mismatches between the crystal lattices of the sapphire substrate and gallium nitride. A technique for forming a low temperature buffer layer on a substrate is one way of suppressing such defects occurring during crystal growth. In a method disclosed in Japanese Patent Laid-open No. Hei 10-312971, general crystal growth is combined with selective crystal growth in the lateral direction (ELO: Epitaxial Lateral Overgrowth) for reducing crystal defects. The method of fabricating a semiconductor light emitting device disclosed in Japanese Patent Laid-open No. Hei 10-312971 has also described that through-dislocations propagated in the direction perpendicular to a principal plane of a substrate are bent in the lateral direction by a facet structure formed in a growth region during fabrication and are thereby prevented from being further propagated, thereby reducing crystal defects.
A light emitting unit, used as an image display unit, can be configured such that pixels, each of which is composed of a combination of light emitting diodes or semiconductor laser devices allowing emission of light of blue, green and red, are arrayed in a matrix, wherein the pixels are independently driven. Such a light emitting unit is also capable of being used as a white light emitting unit or an illumination unit by allowing all of the light emitting devices to simultaneously emit light of blue, green, and red. In particular, since a light emitting device using a nitride semiconductor has a band gap energy ranging from about 1.9 eV to about 6.2 eV, a full-color display unit can be realized by using semiconductor light emitting layers made from only one kind of material. For this reason, research on a multi-color light emitting device using a nitride semiconductor has been actively pursued.
A technique of forming a multi-color light emitting device on the same substrate is also known. Using this technique, a number of active layers having different band gap energies corresponding to different emission wavelengths are stacked, and electrodes on the substrate side are made as a common electrode and electrodes on the other side are individually provided for light of different colors. In particular, it is known that a multi-color light emitting device having a structure with respective steps, which are stepwise formed on the surface side of a substrate for extraction of electrodes, are provided for light of respective colors. However, such a multi-color light emitting device in which a number of pn-junction layers are stacked has a disadvantage in that the light emission regions in the same device may act as a thyristor. To prevent this thyristor action, a multi-color light emitting device has been disclosed, for example, in Japanese Patent Laid-open No. Hei 9-162444, wherein grooves are formed between one and another of stepwise light emission regions for isolating the light emission regions from each other. On the other hand, a light emitting device disclosed in Japanese Patent Laid-open No. Hei 9-92881 is configured such that an InGaN layer is formed on an alumina substrate via an AlN buffer layer, wherein a portion of the InGaN layer is doped with Al to form a blue light emission region, another portion of the InGaN layer is doped with P to form a red light emission region, and a non-doped portion of the InGaN layer is taken as a green light emission region, thereby realizing multi-color light emission.
The above-described methods, however, have the following problems. Namely, the method using selective crystal growth in the lateral direction and the crystal growth method for forming a facet structure in a growth region in order to reduce through-dislocations propagated from a substrate are disadvantageous in that, in a subsequent step, the selective crystal growth in the lateral direction is sufficiently performed or the facet structure is buried in order to form a light emission region such as an active layer. As a result, the number of processing steps is increased and a time required for fabricating the device is prolonged.
The above-described semiconductor light emitting devices for emission of light of multi-colors have the following problems. Namely, since the processing steps become complicated, the light emitting device cannot be accurately formed, and since the crystallinity is degraded, a light emitting device with desirable light emission characteristics cannot be obtained. To be more specific, in the multi-color light emitting device in which grooves are formed between one and another of the stepwise light emission regions for isolating the active layer regions from each other, anisotropic etching must be repeated several times to isolate the active layer regions from each other. Such repeated anisotropic etching is undesirable because the crystallinity of each of the substrate and the semiconductor layer may be generally degraded by dry etching. As a result, it is difficult to keep good crystallinity. Also, the number of steps required for mask alignment and etching is increased. In the multi-color light emitting device in which impurities are selectively doped in the single active layer formed on the substrate, since a margin must be provided for forming an opening portion in the mask layer, a sufficient distance must be set between one and another of the different light emission regions, particularly, in the case of previously estimating a fabrication error, so that it is difficult to form a micro-size light emitting device, and the number of steps is increased by selective doping.
On the other hand, a method is known for fabricating a semiconductor light emitting device in a fine region by forming a layer of a nitride based semiconductor such as GaN into a pyramid shape by selective growth. In particular, a method of fabricating a light emitting device by forming a hexagonal pyramid shaped nitride based semiconductor layer by selective growth has been disclosed, for example, in “Spatial Control of InGaN Luminescence by MOCVD Selective Epitaxy, D. Kapolnek et al., Journal of Crystal Growth, 189/190 (1998) 83-86”. According to the selective growth technique described in this document, a number of nitride based semiconductor light emitting devices, each of which is composed of a fine hexagonal pyramid shaped GaN/InGaN layer structure, can be formed. In a process disclosed in this document, it has been described that an emission wavelength is controlled on the basis of a spacing factor. The document, however, does not describe a definite method for fabricating an image display unit configured such that devices, which allow emission of light of different colors, for example, red, green and blue, are arrayed so as to realize emission of light of multi-colors.